In this work, a numerical study is conducted to investigate the effects of hybrid nanofluid (Al2O3-Cu/water) on the thermal and hydraulic performance of a three-dimensional double-layer counterflow microchannel heat sink. The heat sink comprises a silicon block to which a constant heat flux of q = 1.0 MW/m 2 is applied at the base. Different volume concentrations of alumina and copper nanoparticles are considered, with the Reynolds number varying between 200 and 1000. The conjugate heat transfer problem is solved numerically using the two-phase Eulerian-Eulerian model in ANSYS -Fluent environment. Experimental validation shows a good agreement between the numerical models and the experiment. Nanofluids exhibit higher heat transfer coefficients and pressure drops than the base fluid; however, nanoparticle hybridization has a minimal effect on the pressure drop.